This invention pertains generally to the art of joining aluminum alloy components by brazing and, it pertains in particular to compositions used for vacuum brazing. For purposes of this application the term "vacuum" refers to environments where pressures are below 100 .times. 10.sup..sup.-3 torr and the partial pressures of water and oxidizing species, such as O.sub.2, are below 1 .times. 10.sup..sup.-4 torr and 8 .times. 10.sup..sup.-4 torr, respectively.
As is well known, aluminum alloys containing silicon are appreciated as useful compositions for brazing. Furthermore, it has been recognized that the addition of small amounts of magnesium makes such alloys suitable for vacuum brazing. An unsolved problem has been to find a brazing composition capable of tolerating pressure variations commonly encountered in commercial vacuum brazing operations and effective at relatively mild residual air vacuums on the order of 10.sup..sup.-4, or even 10.sup..sup.-3 torr.
One partially successful solution to the problem has been attained by raising the magnesium content as high as 2.3%, the usual content being less than 1%. (Unless otherwise apparent from the context, percentages given in this application refer to percentages by weight.) The shortcomings of this approach have been found to be two-fold, however. For one thing, the increased level of magnesium tends to frustrate attempts to apply the composition in the form of a cladding and, consequently, results in a higher generation of scrap material. Secondly, magnesium tends to distill or sublime from the composition during brazing, forming deleterious deposits in the brazing chamber.
In accordance with the present invention, it has been found that a brazing composition consisting essentially of aluminum, silicon, magnesium and lead has especially desirable properties for purposes of vacuum brazing. About 0.1% lead is usually sufficient.
The use of lead in lieu of increasing the level of magnesium has been found not only to be substantially free of the drawbacks mentioned in association with the latter alternative; it is also less expensive. For the particular advantages and benefits of the invention, reference is made to the tests and related results and discussion, which are set forth later below.